The technical staff is taking measures to beef up security at the White House servers, he said.

It's not the first time hackers have targeted the U.S. president's Web site, Orr said. "It has happened during the past administration that the Web site was down for a day or two a few times."

Asked whether investigators suspect a link between the incident and a series of recent attacks on U.S. government servers attributed to Chinese hackers, Orr would only say, "I can't speculate at this time."

The U.S. National Infrastructure Protection Center (NIPC), the office of the FBI responsible for computer security, warned late last month of an upswing in attacks against U.S. servers by Chinese hackers, given an increase in political tensions between the countries.

A spokeswoman for NIPC could not be immediately reached for comment Monday.